Maintenance of genomic stability is critical for the well-being of organisms. To maintain genomic stability, cells have developed a network of signaling pathways called the DNA damage response pathway to sense and repair DNA damage. We and others have shown that MDC1 (Mediator of DNA Damage Checkpoint Protein 1, previously known as Kiaa0170), a previously uncharacterized protein, regulates various aspects of the DNA damage response pathway. We have also generated MDC1 knockout mice and shown that cells deficient in MDC1 display genomic instability. These observations support our central hypothesis that MDC1 maintains genomic stability by mediating and facilitating signal transduction pathways following genotoxic stress. We plan to further explore the mechanism of how MDC1 maintains genomic stability. In addition, we will examine the role of MDC1 in tumor suppression using the MDC1 knockout mouse as an animal model. The specific aims are: 1. Explore the biological significance of ATM-dependent phosphorylation of MDC1. We have mapped an ATM phosphorylation site on MDC1, and our preliminary results suggest that this phosphorylation site is involved in the cell cycle checkpoint activation. We will further explore the regulation and functional significance of this phosphorylation site. 2. Investigate the MDC1-topoisomerase II interaction. Our preliminary results suggest that the BRCT domain of MDC1 interacts with phospho-Ser1524 of topoisomerase II, and this interaction regulates the decatenation checkpoint. We will further investigate the regulation of the MDC1-topoisomerase II interaction, and how it regulates the decatenation checkpoint and genomic stability. 3. Investigate the role of genomic instability in aging and tumorigenesis. Genomic instability has been linked to both premature aging and tumorigenesis. We will further evaluate whether loss of MDC1 results in premature aging and tumorigenesis in MDC1-/- mice. Results from these studies will provide new molecular mechanisms of the maintenance of genomic stability and the prevention of aging and tumorigenesis. PUBLIC HEALTH RELEVANCE: Defective DNA damage response pathway is linked to tumorigenesis. Therefore, understanding the DNA damage response pathway will help us understand how cancer arises and how to prevent it. In addition, given that many cancer therapies involve DNA damage-inducing agent, a detailed understanding of the DNA damage response pathway and its defects in cancer cells will help us to design targeted therapy for specific cancers.